Rempt/building krita revisited
Add build guides for Windows, macOS and Android to the building krita page. These guides use the regular scripts we use for every nightly, so this should always work if people install the right prerequisites.
This page should replace 3rdparty/README.md and README.android and any build guides in the community wiki.
Note that the guides for building in docker or with downloaded deps for Windows have gone into a specialized building subfolder so they are harder to find; novices should not use those.